The present disclosure relates to an electrical machine.
Over the past few decades, wind turbines have increasingly been used as energy converters, for example to provide electricity to a national electricity grid. Such wind turbines are considered to utilize renewable energy sources, unlike fossil fuels. It is predicted that fossil fuels may become increasingly expensive and difficult to obtain in the future, and therefore wind turbines are likely to play an increasingly important role for renewable energy.
Wind turbines are often deployed in so-called “wind farms” where a plurality of wind turbines are located within a particular area, for example an area which provides favourable wind conditions in which to operate the wind turbines. Wind farms may often be located in remote locations or situated out at sea so as to minimize disturbance to local inhabitants, and so as to try to provide wind conditions which promote efficient operation of the wind turbines.
Wind turbines typically comprise turbine blades and a generator for generating electricity. In some common designs of wind turbine, the wind turbine comprises a gear box located between the turbine blades and the generator to increase rotational speed sufficiently to be able to drive the generator efficiently. However, reliability of the gear box has been shown to be a problem, especially over the expected operational life span of the wind turbine, which is often more than twenty years. Given that wind turbines are often situated in remote and difficult to access locations, this can create a problem for servicing and maintaining reliability of the wind turbine.
Therefore, there has been recent interest in so-called “direct drive” wind turbines, in which the turbine blades are directly connected to the generator so that a rotor of the generator rotates at the same rotational speed as the turbine blades. The gear box can thus be omitted, saving weight and improving reliability. However, if the gear box is omitted, torque required increases over that required for a generator with a gear box and so the size of the electrical machine needs to be increased. However, as the diameter of the generator is increased, the overall weight of the generator will also increase. However, in most horizontal axis wind turbines, the generator is located within a nacelle at the top of a support tower and during assembly is typically winched into place by a suitable crane. This tends to place an upper practical limit on the weight of the generator of around 100 metric tons.